In recent years, weight reduction of an automotive body has been a crucial issue in the viewpoint of protecting global environments, and studies on the application of a high-strength steel sheet to a vehicle body part have been actively conducted. As the strength of a steel sheet used has been increasing still more, consideration on workability and shape fixability thereof have become important. Further, since the forming load in press forming increases as the strength of steel sheet increases, raising the pressing capability has also become a major issue.
Hot stamp forming (hereafter, also referred to simply as “hot stamping”) is a technique in which a steel sheet is heated to a high temperature in an austenite range and subjected to press forming while it is at the high temperature. Since a softened steel sheet is formed in the hot stamp forming, it is possible to perform more complicated working. Moreover, in the hot stamp forming, since rapid cooling (quenching) is performed at the same timing as the press forming to cause the structure of the steel sheet to undergo martensite transformation, it is possible to achieve strength and shape fixability according to the carbon content of the steel sheet at the same time. Further, since a softened steel sheet is subjected to forming in the hot stamp forming, it is possible to significantly reduce the forming load compared with ordinary press forming which is performed at room temperature.
A hot-stamped part, which is manufactured through hot stamp forming, especially a hot-stamped part used for an automotive body requires excellent low-temperature toughness. A hot-stamped part is sometimes called a steel sheet member. Techniques relating to enhancements of toughness and ductility are described in Patent References 1 to 5. However, the techniques described in Patent Reference 1 to 5 cannot provide sufficient low-temperature toughness. Although Patent References 6 to 10 also disclose techniques relating to hot press forming or the like, they cannot provide sufficient low-temperature toughness as well.